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Calaminarian Grassland

What is it?

A relatively recently-formed habitat, calaminarian grassland is found on old river gravel deposits that were once contaminated by waste from historic mining for lead, silver, zinc, barium, chromium, copper and fluorspar. This waste was released into the rivers and deposited downstream, becoming trapped among the river cobbles.
As the natural river processes progressed, so these shingle areas became exposed. As a result, they were soon colonised by the plants of open grassland which were particularly tolerant to the heavy metals present.

The resulting grassland is varied and often species-rich. Thrift and sea campion accompany grasses grazed by rabbits and sometimes sheep. Lichens and bryophytes are abundant, and bees, hoverflies and butterflies enjoy the nectar provided.

Where is it found?

Outside the UK, calaminarian grassland is very rare. Here, it is a locally widespread, but uncommon, habitat that can be found in the north and west including the Pennines, Cumbria, Wales and Scotland. Near-natural examples exist where rock and mineral veins are exposed with only thin soils, but examples from artificial mine workings and spoil heaps are more common, with several thousand locations across the UK.

Why is it important?

A number of plants almost completely rely on calaminarian grassland such as spring sandwort and alpine penny-cress. A range of common species have become adapted to the unusual conditions including sheep’s fescue, thyme and bladder campion, while several nationally scarce plants grow here including northern rock-cress, forked spleenwort and Young’s helleborine.

It’s thought that the heavy metals present in the soils, combined with a low nutrient status, maintain the open vegetation and stop the grasslands from naturally changing to woodland over time. However, calaminarian grassland is still dependent on the continuous grazing of sheep and rabbits in order to prevent gorse encroaching.

Some sites hold important populations of rare bryophytes and lichens, with as many as 30 lichen species per square metre. Less contaminated sites have a greater diversity of vascular plants, up to 22 per square metre, but fewer lichens.
The flowers attract insects like bees looking for nectar and beetles looking for shelter. In turn, predators such as owls and bats can be seen flying over the grasslands in search of their next meal.

Is it threatened?

In many areas, the metalliferous (metal rich) outcrops which would have been the natural substrate for calaminarian grassland to grow upon have been quarried away, but the mine spoil still provides suitable habitat. Despite this continuation, the species that rely on this kind of habitat have declined rapidly over the past 50 years as many calaminarian grasslands have been lost to mineral extraction or changing land use. On some sites, birch and gorse are invading as the natural succession of vegetation takes hold.

Cattle-grazing also has adverse effects on calaminarian grassland as the increased fertility accompanying their manure can quickly lead to course grasses becoming dominant and shading out fragile flowers. In addition, calaminarian grasslands are fragile habitats which are easily damaged by vehicles and leisure activities.

How are The Wildlife Trusts helping?

The Wildlife Trusts make a significant contribution to the protection of calaminarian grassland by managing nature reserves which contain it sympathetically and by providing advice to landowners on how to look after these fragile places.

What can I do to help?

Take part in conservation measures on your land – ask your local Wildlife Trust for advice on grazing and management methods for calaminarian grasslands.

The Wildlife Trusts are working to protect and restore grasslands for wildlife across the UK, you can support our work by becoming a member of your local Trust

Volunteer with The Wildlife Trusts and help your local grassland wildlife; you could be involved in everything from raising awareness about these rare habitats to scrub-cutting.